Measures For Increasing Power Supply Reliability Of The VSC-HVDC System Connected To Passive Network

HVDC transmission technology based on voltage source converters, using full controlled power electronic devices, is different from conventional HVDC. There is no defects in Commutation Failure, and it can be connected to passive network. VSC-HVDC has already become powerful measures to solve the problem of the urban distribution network expansion.Currently, system modeling research focuses on the steady-state operation, the passive side fault, load shedding and frequency adjustment. But the supply reliability requirement or the important load is getting higher and higher. If there are severe symmetrical, asymmetrical fault and prolonged voltage interruption to the power supply side, both ends of the passive supply system under steady state, is too small to meet the requirements.Therefore, on the passive side, the control strategy of automatic frequency regulation is used and the limit current circle is added. The double-power supply is used in the AC side. Comparing with the existing passive power supply system, it improved the power supply reliability. This method is validated with the PSIM simulation software. It obtains ideal current and voltage and improved the VSC-HVDC system reliability of power supply to the passive network.The main contents and innovations are as follows:First of all,the operating principle and basic structure of the VSC-HVDC system was analyzed, VSC-HVDC system mathematical model under three-phase stationary coordinate system and dq coordinate system was established on the basis of both ends of VSC-HVDC system structure. And by analyzing a variety of fault conditions, a mathematical model of VSC was established.Secondly, the decoupling controller under the steady state and fault conditions was designed.The current loop and the power loop controller was optimized. As parameter adjustment is difficult, small stable work area for the control system of cascade PI regulation, and control method based on feedback linearization simplify was designed to simplify the parameter design. Thirdly, ignoring frequency offset, the synchronized phase of a fault condition, and the symmetrical component of the real-time detection sehemethe were designed. In order to suppress the negative sequence current, the unbalanced control system constituted by the symmetrical structure of the positive sequence current control loop and a current control loop of the negative sequence were designed. Drawing on the grid voltage feedforward control thought, a control strategy based on the imbalance of negative sequence voltage compensation was designed. This control scheme can not only effectively negative sequence current of suppress asymmetric fault, but also greatly simplifies the structure of the control system. The simulation proved that the method improve the performance of the control system.Finally, alter an analysis of the fault condition of the VSC-IIVDC power supply system, the stabilization methods of the DC side voltage were explored. The battery was cut into and voltage control mode is switched to maintain the DC side. For the first time, simplified multiterminal power supply with a dual power supply model were introduced to satisfy the stability requirements of important load under symmetry and asymmetry faults, including single-phase and three-phase ground ibult. A series of simulation validation were carried out. Simulation results verifyed the correctness of the mathematical model and the validity of the controller. They can improve power supply reliability and power quality of VSC-HVDC system to the passive network.